1. Field of the Invention
The present invention relates to a feedforward amplifier and, in particular, to a feedforward amplifier for detecting and suppressing distortion in an output signal amplified by a feedforward loop.
2. Description of the Prior Art
As a linear amplifier employed to simultaneously amplify many frequencies in a high-frequency band, there has been known a feedforward amplifier described in the Japanese Patent Laid-Open Hei-1-198809. FIG. 1 shows an example of structure of the conventional feedforward amplifier. The configuration includes a distortion sensing loop A and a distortion removing loop B such that an input signal containing multiple frequencies received from an input terminal 101 is amplified at a time by a main amplifier 104. The sensor loop A cancels inputted signal components to achieve amplification thereof so as to detect non-linear distortion components caused by the amplification. The remover loop B amplifies the detected distortion components by an auxiliary amplifier 115 and adds the resultant signal to an output signal from the main amplifier 104, thereby cancelling the distortion components.
The distortion sensor loop A includes a pilot oscillator 118, a (directional) coupler 119 for superimposing a pilot signal onto an input signal, a two-output distributor 102, a vector adjuster 103 for adjusting attenuation and phase shift, a main amplifier 104, a delay line 106, a coupler 110, a detector (DET) 122, and a control circuit 109. Additionally, couplers 107 and 108 are arranged as shared components for the sensor loop A and remover loop B. The coupler 108 produces an output signal, which is fed via the coupler 110 to the detector 122 to be detected therein. The detected signal is applied to the control circuit 109. The controller 109 supervises the vector adjuster 103 to set the output level of the detector 122 to a minimum value.
Furthermore, the distortion remover loop B includes the couplers 107 and 108, which are also used by the distortion sensor loop A, a pilot oscillator 120, a coupler, a delay line 111, a coupler 112, a vector adjuster 113 for regulating attenuation and phase shift, an auxiliary amplifier 115, a coupler 116, a detector (DET) 123, and a control circuit 114.
In the output circuit of the feedforward circuit, the control circuit 114 detects the pilot signal via the coupler 116 and detector 123 to control the vector adjuster 113 so as to set the sense level of the pilot signal to a minimum value.
Description will now be given of operation of the conventional linear (feedforward) amplifier constructed as above. A multiplex signal containing many frequencies in a high-frequency band is received via the input terminal 101 to be superimposed by the coupler 119 onto the output signal from the pilot oscillator 118. The obtained signal is distributed by the distributor 102 as a first signal and a second signal.
The first signal is fed to the vector adjuster 103. Attenuation and phase shift thereof are regulated by the adjuster 108. The resultant signal is delivered to and amplified by the main amplifier 104 to be coupled by the coupler 121 with the pilot signal generated from the pilot oscillator 120. The obtained signal is fed via the coupler 107 and delay line 111 to the coupler 112. The signal fed to the coupler 112 is called a main amplification signal including distortion components caused during the amplification by the main amplifier 104.
The second signal thus distributed by the distributor 102 is delayed through the delay line 106 by a delay time equivalent to that of the vector adjuster 108 and main amplifier 104 to be fed to the coupler 108. On the other hand, a portion of the main amplification signal is branched by the coupler 7 to be supplied to the coupler 108 in an inverse-phase state. These signals are mixed with each other by the coupler 108 to be supplied to the coupler 110. The pilot signal contained in the portion of the signal branched by the coupler 110 is detected by the detector 122 to be inputted to the controller 109. The control circuit 109 adjusts attenuation and phase shift of the vector adjuster 108 to minimize the output signal level of the detector 122.
In the construction, the output terminal of the distributor 102 is linked with the coupler 108 only by the delay line 106 and hence distortion generated by the line 106 is negligible. Consequently, if the sensor loop A is appropriately operating, the portion of the input signal passed through and amplified by the vector adjuster 108 and main amplifier 104 is coupled by the coupler 108 with the first signal in an inverse-phase state. Resultantly, only the distortion component generated or mixed in the main amplifier 104 are outputted from the coupler 108.
The distortion component signal fed from the coupler 108 to the coupler 110 is then delivered to the vector adjuster 118 for adjustment of attenuation and phase shift thereof. The obtained signal is then supplied to and amplified by the auxiliary amplifier 115. The amplified distortion component is the mixed by the coupler 112 in the reverse-phase state with the main amplification signal delayed through the delay line 111 by a propagation delay time of the vector adjuster 113 and auxiliary amplifier 115.
The composite signal is then supplied to the coupler 116 such that a portion thereof is branched to the detector 128. The detector 123 conducts, for example, a simultaneous detection for the inputted composite signal to extract therefrom a pilot signal and then sends the pilot signal to the control circuit 114. The controller 114 regulates attenuation and phase shift of the vector adjuster 118 to set the output level of the detector 128 to a minimum value. As a result, in the main amplification signal sent from the coupler 116 to the output terminal 117, the pilot signal takes the minimum value, namely, the distortion component is minimized.
The signal fed to the input terminal 101 is subjected to modulation due to information transmission and may possibly continually interrupted in some cases. Consequently, when the input signal is adopted to control amplification, the amplified signal becomes unstable. However, according to the conventional linear (feedforward) amplifier configured as above, since amplification is supervised by a pilot signal of which the level and frequency are determined, there can be attained a stable amplification signal.
However, in the linear amplifier of the prior art, to obtain compression of, for example, at least 30 decibel (dB) in a frequency band, it has been known in computation that the deviations respectively of amplitude and phase are required to be within .+-.0.3 dB and .+-.2.degree., respectively. These values indicate deviations respectively thereof related to the frequency used and the frequency of the pilot signal when the pilot signal is completely canceled. Consequently, in the conventional amplifier, a stable control operation is expected in the neighborhood of the frequency of the pilot signal. However, to remove the 30 dB distortion uniformly in any zone of a frequency band, the constituent elements of the loops included in the main and auxiliary amplifiers 104 and 115 are required to have severe frequency characteristics, namely, the deviations respectively of amplitude and phase are required to be within .+-.0.3 dB and .+-.2.degree., respectively.
In consequence, the linear amplifier of the prior art is attended with a problem that the amplifier is unsuitable for amplification of signals in a wide-band.